The invention relates generally to the field of magnetic bubble technology (MBT) and more particularly to logic arrangements utilizing the capabilities of single wall magnetic domain devices.
The continuing evolution of MBT has now reached the point where large scale application to various data processing is practicable. Current interest in MBT is due primarily to the prospect of extremely high bit packing density, low power comsumption and reliability for low cost mass memories.
Briefly, MBT involves the creation and propagation of single wall magnetic domains in specially prepared magnetic materials. The application of a static uniform magnetic bias field orthogonal to a sheet of magnetic material having suitable uniaxial anisotropy causes the normally random serpentine pattern of magnetic domains to contract or shrink into short cylindrical figurations or bubbles whose common polarity is opposite that of the bias field. The bubbles repel each other and can be moved or propagated by a magnetic field in the plane of the sheet.
Many schemes now exist for propagating the bubbles along the sheet in predetermined channels. One propagation system includes permalloy circuit elements shaped like military uniform stripes or "chevrons" spaced end-to-end in a thin layer over the sheet of magnetic material. The drive or propagation field is continuously rotating in the plane of the sheet causing each chevron to act as a small magnet whose poles are constantly changing. As the drive field rotates, a bubble under one of the chevrons is moved along the chevron channel from point to point in accordance with its magnetic attraction to the nearest attracting temporary pole of the circuit elements. This system is among those referred to as "field-access" as distinguished from other systems employing loops of conductors disposed over the magnetic sheet.
The use of MBT in data processing stems from the fact that the bubbles can be propagated through their channels at a precisely determined rate so that uniform data streams of bubbles are possible in which the presence or absence of a bubble indicates a binary 1 or 0. The use of MBT for performing logic operations is based on the fact that close magnetic bubbles tend to repel each other. Thus, if alternate paths with varying degrees of preference are built into the chevron circuit, the direction which a bubble on one channel ultimately takes may be influenced by the presence or absence of a bubble on another closely spaced channel. Logic systems capitalizing on this principle are shown in U.S. Pat. Nos. 3,678,287 and 3,676,871 to Chow and Copeland, respectively. An additional logic "interactor" is shown in the June 1971 issue of Scientific American in an article beginning on page 78 entitled "Magnetic Bubbles" by Bobeck and Scovil. A paper dealing with this topic was also presented at the American Institute of Physics conference proceedings, No. 5, pp. 45- 55 (1972), entitled "An Overview of Magnetic Bubble Domains - Material Device Interface" by Bobeck, Fisher and Smith. Another recent article on this topic is "Logic Functions for Magnetic Devices" by Sandfort and Burke, IEEE Trans. Mag., Vol. MAG-7, No. 3, Sept. 1971, pp. 358- 360.
Besides the inherent capability of performing logic with magnetic domains, one other aspect of MBT has given impetus to logic development. MBT was originally envisioned as a mass memory but the most difficult problem has been encountered in readout. Optical devices utilizing the Faraday effect and magnetoresistive devices have been used, but are not entirely satisfactory. Therefore, it is important to minimize readout to the extent possible by incorporating logic in the memory so that the magnetic bubbles representing information can be logically manipulated before readout is necessary.
In the above Bobeck, Fisher and Smith article, a single three input, three output (3--3) logic gate was specified (hereafter the Bobeck gate). The gate structure involved the principle of uniformly graded packing density of identical permalloy chevrons between tracks in a direction transverse (vertical) to normal propagation. Structurally, the Bobeck gate utilized three approximately parallel bubble channels formed by chevron tracks interconnected by a vertical section several chevrons long having chevrons extending in columns across all three channels. The vertical spacing between chevrons decreases toward the middle channel. The gate has three inputs to receive three variables X, Y and Z, where each of these variables can represent 0 and 1 (i.e. bubble or no bubble) and three output functions f.sub.1, f.sub.2, f.sub.3 where f.sub.1 = f.sub.3 = XY + XZ + YZ and f.sub.2 = X.sym.Y.sym.Z.